Python parsing variable length structure

Recently, I received a request to use Python to parse the data packet from C, and the format of the data packet is defined by the following structure:

typedef struct msg_t
{
 int oid;
 int msg_len;
 char msg_data[0];}MSG_T;

The length of the msg_data string is given by msg_len, so you need to parse out the value of msg_len first, and then read the content of msg_len.

This operation can be done through the struct module in Python. The python parsing code for the above data structure is as follows:

  OID =0
 msgLen =0
 msgData =""
 sFormat =""

 OID, msgLen = struct.unpack('II', syncMsg[0:8])
 sFormat ='II'+str(msgLen)+'s'
 OID, msgLen, msgData = struct.unpack(sFormat, syncMsg)
 msgData = msgData.decode()
 # print("OID: ", OID,"\nMsgLen: ", msgLen,"\nMsgData: ", msgData.decode())

The core of the code is the single quote part when unpack, where I represents Int, and 128s represents a string of length 128. Here, the length is analyzed first, and then the data format is spliced, and then analyzed.

The supported formats in struct are as follows:

Format	C Type	Python	Bytes
x	pad byte	no value	1
c	char	string of length 1	1
b	signed char	integer	1
B	unsigned char	integer	1
?	_ Bool	bool	1
h	short	integer	2
H	unsigned short	integer	2
i	int	integer	4
I (uppercase i)	unsigned int	integer or long	4
l (lowercase L)	long	integer	4
L	unsigned long	long	4
q	long long	long	8
Q	unsigned long long	long	8
f	float	float	4
d	double	float	8
s	char[]	string	1
p	char[]	string	1
P	void *	long	4

references##

Analysis of the struct module in Python: https://www.cnblogs.com/coser/archive/2011/12/17/2291160.html
python struct structure: https://blog.csdn.net/CLinuxF/article/details/102478001
The operation of byte stream/binary stream in Python: easy to use struct module tutorial: https://www.jianshu.com/p/5a985f29fa81
struct --- Interpret the byte string as packed binary data: https://docs.python.org/zh-cn/3/library/struct.html

Author: Frytea
Title: Python parsing variable length structure
Link: https://blog.frytea.com/archives/453/
Copyright: This work by TL-Song is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.